Temperature measurements along a vertical borehole heat exchanger: A method comparison

The standard thermal response tests (TRT) provide integral and effective thermal parameters of the ground in the vicinity of borehole heat exchangers (BHE). However, typical ground properties are heterogeneously distributed. As a result, advanced TRT such as distributed and enhanced TRT are growing in popularity as they provide more spatial information of the thermal properties. Thus, the objective of this study is to compare various instruments to measure the depth-dependent temperatures using standard Pt100-sensors, fiber optical thermometers and novel instruments such as Geowire, Geoball and GEOsniff®. The investigations are carried out in a 30 m length test borehole. The results showed an excellent agreement between both the Geowire and GEOsniff® in comparison with Pt100-sensors with a root mean squared error of 0.10 and 0.09 K, respectively. The results also suggest that the novel instruments have various advantages over the standard sensors and fiber optics. For example, with the novel instruments comparable, accurate, inexpensive, instantaneous and higher spatial resolution temperature measurements are obtained. Finally, the outcome of this study provides a guide for choosing the adequate temperature measurement along a BHE thus generally improving the evaluation of advanced TRT, while potentially increasing efficiency and economic viability of ground-source heat pump systems.